In recent years, the facts that fine metal particles having a particle size on the nanometer level (hereafter referred to as metal nanoparticles) exhibit physical properties different from those of normal bulk metals have been continuously revealed. Novel materials employing such properties are being intensively developed. In particular, attempts to use metal nanoparticles as conductive printing materials employing the phenomenon that metal nanoparticles have a fusion temperature much lower than that of bulk metal have been widely made. For example, although silver normally has a melting point of more than 960° C., a phenomenon has been observed that nanoparticles thereof having a size of 100 nm or less readily fuse together even at a low temperature of about 200° C. Accordingly, when an ink containing metal nanoparticles exhibiting low-temperature fusibility is prepared, good electrical wiring can be formed by only drawing a circuit by a printing process and then sintering the circuit at a low temperature. This novel circuit-formation method is markedly simple and economical, compared with photolithography in which physical and chemical treatments such as masking and etching treatments are repeatedly performed.
Under such circumstances, practical use of gold nanoparticles and silver nanoparticles as conductive materials has evolved. In addition, nanoparticles of noble metals such as palladium, rhodium, and platinum that are excellent in catalysis are being used in wider applications. Compared with such nanoparticles, copper nanoparticles may be produced at low cost because copper compounds serving as raw materials are inexpensive. However, compared with other noble metals, as for copper, it is difficult to control particle size on the nanometer level or to ensure dispersion stability. In addition, copper is very susceptible to oxidation. Accordingly, the development of copper nanoparticles has not sufficiently advanced.
On the other hand, there is a method in which partial oxidation of copper is regarded as being unavoidable and oxidized copper is reduced during sintering. Specifically, various physical techniques have been developed: for example, a technique in which a dispersion liquid of copper nanoparticles is applied to a substrate to form a thin film or to draw a conductive pattern, and a wiring pattern is then completed under a reducing atmosphere such as hydrogen gas, ammonia, carbon monoxide, atomic hydrogen, or alcohol vapor; and a technique in which metal nanoparticles are fused together by high-frequency irradiation to form a porous conductive thin film. Together with developments of apparatuses such as a microwave hydrogen-plasma generator and a pulse photon emitting apparatus, various techniques have been provided for the purpose of achieving practical use of a dispersion of copper nanoparticles. A stable dispersion of copper nanoparticles has been demanded.
Fine copper particles have been synthesized since a long time ago by a method (polyol reduction method) in which a copper salt, a copper oxide, or the like is treated at a high temperature in a polyhydric alcohol having a high boiling point. However, this method tends to provide relatively large particles having a size of several hundred nanometers to about one micrometer and the particles do not have the low-temperature sinterability that is expected for electronic materials.
Afterwards, synthesis of reduced metal nanoparticles having a size of 100 nm or less was achieved by adding, to the reaction system, a polymer substance such as polyvinyl pyrrolidone or an amine-based organic compound. Thus, the phenomenon that fusion occurs at a low temperature of 300° C. or less can be expected, and application of the particles to an efficient printing technique, an inkjet process, has come to be realized. Such polyvinyl pyrrolidone and an amine-based compound are compounds having a function of adhering to generated metal nanoparticles to suppress an increase in the particle size and a function of stably maintaining and dispersing generated metal nanoparticles in a medium. These compounds are referred to as capping agents and colloid protective agents. These are indispensable for formation of metal nanoparticles and also serve as factors dictating characteristics such as fusion temperature, and thus are the important technical component for practical use of metal nanoparticles.
Such a metal-colloid protective agent needs a structure that is compatible with a solvent and imparts a dispersion stabilization function through exhibition of repulsion in response to the close presence of metal nanoparticles, and a partial structure that has an affinity allowing adsorption of the structure onto the metal nanoparticles. These structures may be the same structure or different chemical structures. In consideration of dispersion in an aqueous medium, the former structure may be a hydrophilic polymer structure such as polyethylene glycol, a copolymer between polyethylene glycol and polypropylene glycol, polyethyleneimine, polyacrylamide, polyvinyl acetate, polyvinyl alcohol, or polyvinyl pyrrolidone. When the medium is an organic solvent, a hydrophobic functional group such as a long-chain alkyl group or a phenyl group is used.
On the other hand, the affinity for metal surfaces is provided by a hetero functional group. Specifically, a lone electron-pair in a hetero atom is coordinated to a metal ion or the surface of a metal nanoparticle to thereby achieve the adsorption. Examples of commonly used adsorptive functional groups include —OH, —O—, —SH, —CN, —NH2, —NR3, —SO2OH, —SOOH, —OPO(OH)2, and —COOH. In particular, compounds having a —SH group or a —COOH group have a very high affinity for copper and copper compounds and are used in many cases (for example, refer to Patent Literature 1).
In general, coordination of the thiol functional group (—SH) to the surface of a metal nanoparticle has a very high strength that is similar to a covalent bond. Accordingly, when a thiol compound is used as a protective agent, the protective agent having coordinated is not easily dissociated. For this reason, in general, the protective agent is dissociated by heating to the decomposition temperature of the thiol functional group. Thus, use of nano metal as a conductive material in which a thiol compound is used as the protective agent exerts large detrimental effects. On the other hand, it is known that the thioether (C—S—C) functional group can coordinate to metal, but this coordination has a weak strength; and the group is considered to have a high affinity for copper nanoparticles (for example, refer to Patent Literature 2). However, there have been no cases where the group is actually incorporated as an affinity functional group into a colloid protective agent and used for producing copper nanoparticles or copper(I) oxide nanoparticles.
Citation List
Patent Literature
                PTL 1: Japanese Unexamined Patent Application Publication No. 2004-143571        PTL 2: Japanese Unexamined Patent Application Publication No. 2004-119686        